1. Technical Field
The present invention relates to a technique of compressing image data composed of a plurality of pixels and a technique of decoding and outputting the compressed image data.
2. Related Art
In general, when outputting an image, such as a natural image, having smooth density variation by using an image output apparatus such as a printer, an image quality sufficient for common use can be obtained if the image is output with resolution of about 360 dpi×360 dpi. However, when printing an image, such as a character or a symbol, having abrupt density variation, edges of the image are blurred or the line thickness becomes non-uniform in the case of resolution of about 360 dpi×360 dpi, which causes the image quality to deteriorate.
Therefore, in order to secure the image quality of an image including a character or a symbol, it may be considered to output the image with the resolution of 720 dpi×720 dpi, for example. However, in this case, since the data capacity of image data increases, it is necessary to increase the capacity of a memory mounted in the image output apparatus as much as the increased data capacity. In addition, according to the increase of the data capacity, an interface by which the image data is transmitted from an image processing apparatus, such as a computer, to an image output apparatus needs to be adapted to high speed.
In connection with the above-mentioned problems, for example, in Japanese Patent Publication No. 7-87315A (JP-A-7-87315), the resolution is dynamically changed according to the complexity of a pixel configuration of a part of image data, thereby reducing the data capacity.
Furthermore, in Japanese Patent Publication No. 4-356873A (JP-A-4-356873), image data is subjected to various image processing, such as MTF correction or white background extraction, and a character region and a natural image region are separated from each other. Then, different compression methods are applied to these regions, thereby compressing the image data at high compression rate. Specifically, a reversible entropy coding processing is executed for the character region and a irreversible ADCT (adaptive discrete cosine transform) coding processing is executed for the natural image region.
However, in JP-A-7-87315 described above, depending on whether or not a predetermined pixel pattern continues, the complexity of a corresponding region is determined. Accordingly, a complex operation processing should be executed until the resolution switches.
On the other hand, in JP-A-4-356873 described above, it is difficult to determine the separation between the character region and the natural image region. In addition, since different compression methods are applied to these regions, two different kinds of hardware that performs a compression and two different kinds of hardware that performs a decoding corresponding to the compression methods are needed, which causes the configuration of a device to be complicated.